IxXX BEPORT 1870. 



of the air, nor the composition of the organic matter, ■was altered, in such a 

 manner as to interfere with the existence of life. 



Schulze and Schwann took up the question from this point of view in 

 1836 and 1837. The passage of air through red-hot glass tubes, or through 

 strong sulphuric acid, does not alter the proportion of its oxygen, while it 

 must needs arrest, or destroy, any organic matter which may be contained in 

 the air. These experimenters, therefore, contrived arrangements by which 

 the only air which should come into contact Avith a boiled infusion should be 

 such as had either passed through red-hot tubes, or through strong sulphuric 

 acid. The result which they obtained was that an infusion so treated deve- 

 loped no living things, while if the same infusion was afterwards exposed to 

 the air such things appeared rapidly and abundantly. The accuracy of these 

 experiments has been alternately denied and aiFirmed. Supposing them to 

 be accepted, however, all that they really proved was, that the treatment to 

 which the air was subjected destroyed something that was essential to the 

 development of life in the infusion. This " something " might be gaseous, 

 fluid, or solid; that it consisted of germs remained only an hypothesis of 

 greater or less probability. 



Contemporaneously with these investigations a remarkable discovery was 

 made by Cagniard de la Tour. He found that common yeast is composed of 

 a vast accumulation of minute plants. The fermentation of must, or of wort, 

 in the fabrication of wine and of beer, is always accompanied by the rapid 

 growth and multiplication of these Toridce. Thus fermentation, in so far as 

 it was accompanied by the development of microscopical organisms in enormous 

 numbers, became assimilated to the decomposition of an infusion of ordinary 

 animal or vegetable matter ; and it was an obvious suggestion that the 

 organisms were, in some way or other, the causes both of fermentation and of 

 putrefaction. The chemists, with Berzelius and Liebig at their head, at first 

 laughed this idea to scorn; but in 1843, a man then very young, who has 

 since performed the unexampled feat of attaining to high eminence alike iu 

 Mathematics, Physics, and Physiology, — I speak of the illustrious Helmholtz 

 — reduced the matter to the test of experiment by a method alike elegant 

 and conclusive. Helmholtz separated a putrefying, or a fermenting liquid, 

 from one which was simply putrescible, or fermentable, by a membrane, which 

 allowed the fluids to pass through and become intermixed, but stopi)ed the 

 passage of solids. The result was, that while the putrescible, or the 

 fermentable, liquids became impregnated with the results of the putrescence, 

 or fermentation, which was going on on the other side of the membrane, they 

 neither putrefied (in the ordinary way) nor fermented ; nor were any of the 

 organisms which abounded in the fermenting, or putrefying, liquid generated 

 in them. Therefore, the caiise of the development of these organisms must 

 lie in something which cannot pass through membrane; and as Helmholtz's 

 investigations were long antecedent to Graham's researches upon colloids, his 

 natural conclusion was, that the agent thus intercepted must be a solid ma- 

 terial. In point of fact, Helmholtz's experiments narrowed the issue to this : 

 that which excites fermentation and putrefaction, and at the same time gives 

 rise to living forms iu a fermentable, or putrescible, fluid, is not a gas and is 

 not a diff'usiblo fluid ; therefore it is either a colloid, or it is matter divided 

 into very miniite solid particles. 



The researches of Schroeder and Dusch in 1854, and of Schroeder alone in 

 1859, cleared wp this point by experiments which are simply refinements 

 upon those of Eedi. A lump of cotton-wool is, physically speaking, a pile of 

 many thicknesses of a very fine gauze, the fineness of the meshes of which 



